Water-swellable polymers are us,ed as constituents in personal care devices which absorb bodily fluids, such as sanitary napkins, incontinent devices and disposable baby diapers. Examples of such polymers are disclosed in U.S. Pat. Nos. 3,926,891; 4,190,562; and 4,293,609.
Various processes are known for producing absorbent polymers. For example, U.S. Pat. No. 4,833,222 teaches a process for preparing an absorbent polymer from a neutralized monomer with a surface-active agent. U.S. Pat. No. 4,808,637 teaches the uniform reaction of acrylic acid, an alkali metal salt of carbonic acid, aluminum acetate, sodium sulfate and water, preferably using microwave radiation as a heat source to initiate polymerization.
Another process comprises the steps of preparing a reaction mixture consisting essentially of particular amounts of polymerizable acid group-containing monomers, crosslinking agent and optionally free radical initiator in an aqueous medium and thereafter polymerizing and neutralizing at least a portion of the acid functional groups.
Absorbent polymers used in personal care devices are characterized by certain performance properties, including centrifuge capacity, absorbence under load, shear modulus, and percent extractables.
U.S. Pat. No. 4,666,983 discloses an absorbent article obtained by mixing 100 parts by weight of an absorbent resin powder having a carboxyl group with 0.0001 to 10 parts by weight of a crosslinking agent to crosslink the molecular chains existing at least in the vicinity of the surface of the absorbent resin powder.
U.S. Pat. No. 4,734,478 discloses a water-absorbing resin powder having the molecular chains near its surface, the resin powder being obtained by mixing 100 parts by weight of a powder of a carboxyl-containing water-absorbing resin with 0.001 to 10 parts by weight of a polyhydric alcohol and heating the mixture at a temperature of at least 100.degree. C. to react the powder with the polyhydric alcohol, the improvement comprising conducting the mixing of the powder and the polyhydric alcohol in the presence of 0.01 to 8 parts by weight of a hydrophilic organic solvent and 0 to 8 parts by weight of water per 100 parts by weight of the powder.
U.S. patent application Ser. No. 07/866,628 discloses a process for preparing a surface crosslinked water absorbent polymer, wherein a hydrogel is contacted with a composition containing a polyhydroxy compound and a surfactant to coat the hydrogel. The coated hydrogel is then dried to form particles, is optionally ground to form sized particles, and is heated to crosslink the surface of the particles. In preferred embodiments, the hydrogels will comprise from about 15 to about 100 weight percent polymer, with the remainder comprising water. In preferred embodiment, the hydrogel is preferably in granular form, with particle sizes of about 2 cm or less being more preferred. The composition which contains the polyhydroxy compound and the surfactant will preferably contain from about 0 to about 99 percent water and from about 0 to about 50 weight percent organic solvent.
Japanese Kokai Publication 84632/1981 discloses an absorption agent consisting of a crosslinked poly(alkali metal acrylate) material obtained from 0.01 to 10 parts by weight of a water-soluble and/or water dispersible surfactant and 0.005 to 20 parts by weight of a water-soluble polyvalent alcohol per 100 parts by weight of an acrylic acid/acrylate based polymer composed of from 10-40 mole percent acrylic acid and 60-100 mole percent acrylate salt, wherein the polymer is formed by gel polymerization as a gel which is subsequently heated and dried.
European Patent Application 248,963 discloses a process of post treatment wherein the surface of water-absorbent resin particles are treated with polyquarternary amines to significantly increase the absorption rate and to increase the absorption under load (AUL) by 10 percent. In the disclosed process, the polyquarternary amines are applied as solutions in methanol.
European Patent Application 248,437 discloses a process for post surface crosslinking in which an aqueous solution of a water-soluble peroxide radical initiator is sprayed onto the surface of the water absorbent resin particles and the coated particles are heated. The reference alleges the achievement of additional surface crosslinking, leading to a product of improved water absorbency and water absorption rate. The reference discloses that the uniformity of penetration of the aqueous solution into the surface of the absorbent polymer may be improved by using a water soluble organic solvent such as methanol. The disclosed process suffers the disadvantage of using high levels of peroxide free radical initiators, which discolors partly neutralized polyacrylic acid, making the product less appealing for personal care applications. The disclosed process suffers the further disadvantage of increasing the amount of extractable polymer, e.g., low molecular weight water-soluble polymer, if the polymer is subjected to heat.
German Patent DE 3,713,601 discloses a process in which surface crosslinking is obtained by the addition of a crosslinker of glycidyl or polyglycidyl compounds. These crosslinking agents are not preferred in applications wherein the polymer is expected to contact human skin.
Industry would find great advantage in an aqueous fluid absorbent having improved absorbency under load and capacity. Industry would further find great advantage in a process for producing such an aqueous fluid absorbent